Conventionally, expander-integrated compressors are known as a fluid machine having a compression mechanism and an expansion mechanism. FIG. 29 shows a vertical cross-sectional view of an expander-integrated compressor described in JP 2005-299632 A.
An expander-integrated compressor 103 includes a closed casing 120, a compression mechanism 121, a motor 122, and an expansion mechanism 123. The motor 122, the compression mechanism 121, and the expansion mechanism 123 are coupled to each other with a shaft 124. The expansion mechanism 123 recovers mechanical power from a working fluid (for example, a refrigerant) that is expanding, and supplies the recovered mechanical power to the shaft 124. Thereby, the power consumption of the motor 122 driving the compression mechanism 121 is reduced, improving the coefficient of performance of a system using the expander-integrated compressor 103.
A bottom portion 125 of the closed casing 120 is utilized as an oil reservoir. In order to pump up the oil held in the bottom portion 125 to an upper part of the closed casing 120, an oil pump 126 is provided at a lower end of the shaft 124. The oil pumped up by the oil pump 126 is supplied to the compression mechanism 121 and the expansion mechanism 123 via an oil supply passage 127 formed in the shaft 124. Thereby, lubrication and sealing can be ensured for the sliding parts of the compression mechanism 121 and those of the expansion mechanism 123.
An oil return passage 128 is provided at an upper part of the expansion mechanism 123. One end of the oil return passage 128 is connected to the oil supply passage 127 in the shaft 124, while the other end opens downward below the expansion mechanism 123. Generally, the oil is supplied excessively in order to ensure the reliability of the expansion mechanism 123. The excess oil is discharged below the expansion mechanism 123 via the oil return passage 128.
The amount of the oil mixed in the working fluid in the compression mechanism 121 usually is different from that in the expansion mechanism 123. Accordingly, in the case where the compression mechanism 121 and the expansion mechanism 123 are accommodated in separate closed casings, a means for adjusting the oil amounts between the two closed casings is necessary in order to prevent excess and deficiency of the oil. In contrast, the expander-integrated compressor 103 shown in FIG. 29 substantially is free from the problem of excess and deficiency of the oil because the compression mechanism 121 and the expansion mechanism 123 are accommodated in the same closed casing 120.
In the above-mentioned expander-integrated compressor 103, the oil pumped up from the bottom portion 125 is heated by the compression mechanism 121 because the oil passes through the compression mechanism 121 having a high temperature. The oil heated by the compression mechanism 121 is heated further by the motor 122, and reaches the expansion mechanism 123. The oil having reached the expansion mechanism 123 is cooled in the expansion mechanism 123 having a low temperature, and is discharged below the expansion mechanism 123 via the oil return passage 128. The oil discharged from the expansion mechanism 123 is heated when passing along a side face of the motor 122, and is heated further when passing along a side face of the compression mechanism 121. The oil then returns to the bottom portion 125 of the closed casing 120.
As described above, the oil circulation between the compression mechanism and the expansion mechanism causes heat transfer from the compression mechanism to the expansion mechanism via the oil. Such heat transfer lowers the temperature of the working fluid discharged from the compression mechanism, and raises the temperature of the working fluid discharged from the expansion mechanism, hindering improvement of the coefficient of performance of the system using the expander-integrated compressor.